I'm not selling any bitcoins to the merchant. I am just paying the merchant with bitcoins directly to buy a product. (the merchant processes my payment through bitpay)
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So buying something with bitcoins as I explained above is considered "send" right? The bitcoins->USD conversion which is done by bitpay is done at the merchant side right?
No, not taxable.
That is not correct for the U.S.
If you used bitcoin by cashing it on an exchange or buying goods and services, you will owe taxes if the realized value (the sale price of bitcoin, for example) is greater than the price at which you acquired the bitcoin. You may have a capital gain that's taxable at either short-term or long-term rates.
That is mostly correct. In the U.S, when you exchange bitcoins for anything, whether it is dollars, goods, or even other coins, that is a taxable event -- a capital gain or a loss.
This explains everything:
https://www.irs.gov/individuals/international-taxpayers/frequently-asked-questions-on-virtual-currency-transactions